I guess a private CPAN might be useful in a network of disparate systems. I'm fortunate to not work in such an environment. The only time I download something from CPAN is when I'm building a .deb from it. This activity would take place on a dev server and never on a production server. The advantage of using the system's native packaging is that you can set up dependencies on things that aren't CPAN modules and deployment is a breeze.

I'm not suggesting your proposal is a bad idea - just not something I'd use.